The IP is fetched with methods from the Request class that you cannot mess around it. You could run some PHP code to change the $_SERVER environment in /conf/bootstrap.before.php but that is pointless.

Your server logs all IP addresses and passes them over to PHP. If you want to grant your users that their IP is not logged, you should use apaches SetEnv and dontlog features to never show any "real" IP. I don't think it will be a problem if all users are coming from one IP address, but you would have to give that a short try before you go into production with such a tweak.

Please recognize that the timeline for something like this is likely "next year or beyond", not something I can do real quick before 2.7 or something. IP addresses are a foundational assumption in the software so I'm mildly terrified at the idea of how much work this could potentially be.

As I've said before: doing this on the PHP level is the wrong approach since client/server communication is handled on the server. The moment the IP is handled from the server over to the script is where you have failed if you want to grant maximum anonymity.

But I never was a fan of only questioning people and telling them they are using the wrong approach. If I can, I show how to follow that wrong path

In this case, I would assume it is enough to patch class.request.php. Line 757 shows following code:

should make your server believe that every user has the same IP like the Bundesamt für Sicherheit in der Informationstechnik

Beware! Some scripts might use the IP to differentiate between users, which would be wrong. And if you like to use an IP that is reserved for localhost or local networks, you might open your forum up for security issues.